The present invention relates to a light transmitting panel of the type composed of at least two panes held in spaced relation by one or more spacer members to define at least one inter-pane space. The invention also relates to a method of manufacturing such a panel. The invention relates in particular to panels having improved acoustic properties and, in addition, improved thermal properties.
The use of large areas of windows and other panels, such as glazed partitions, which is a feature of modern architectural practice, gives rise to the problem of achieving comfortable sound levels in rooms in whose walls the panels are installed, especially in noisy environments. The problem is particularly acute in the cases of windows facing busy roads or near airports, and light transmitting panels having good acoustic insulation properties are required in these cases, as well as in the case of interior partitions such as in sound recording and broadcasting studios.
Light transmitting panels composed of two or more sheets of glass or plastic material held in spaced relation by one or more spacer members such as have been made for use as windows with the object of reducing heat loss from a building also give rise to a loss in sound transmission, i.e. a rise in sound insulation, but this sound transmission loss, is, in general, insufficient for many purposes.
This sound transmission loss can be increased by increasing the width of the or each inter-sheet space, but this gives rise to manufacturing difficulties and increases the cost of the panel: it also involves the use of a larger and therefore heavier and more expensive frame for holding the panel in position.
It has also been proposed to vary the masses of the sheets of the panel and the ratios of the masses of the different sheets, in order to improve the acoustic properties of the panel.
When plotting a graph of sound transmission loss through a given panel against various frequencies of incident audible sound, it will be found that the result is not a straight line and that there are various regions where transmission peaks occur.
One such transmission peak occurs at fairly high audible frequencies and is due to the so-called coincidence effect. The frequency of the sound waves giving rise to the coincidence effect at a given sheet depends upon the angle of incidence of such waves on the sheet and corresponds to the frequency at which the projected incident wavelength on the sheet is equal to the wavelength of free bending waves in the sheet. Thus the lowest sound frequency at which coincidence takes place, the critical frequency, is that which corresponds to a sound wavelength equal to the free bending wavelength. The free bending wavelength of a sheet, according to currently accepted theories, decreases with increasing thickness, i.e. with increasing mass per unit surface area. It is known to reduce the coincidence transmission peak across a multiple glazing unit by making the sheets of different masses, so that the coincidence peak for one sheet falls at a different frequency from that of another sheet.
Another such transmission peak occurs at a fundamental resonance frequency of the panel, and this also depends inter alia on the masses of the sheets. For a single sheet of a given area it has been calculated that the resonance frequency increases with the mass of the sheet. In a multi-sheet panel, the sheet spacing also has an effect on the resonance frequency.
In the median range of audible sound frequencies, that is between the coincidence and resonance transmission peaks, the sound transmission loss increases with increase in the total mass of the sheets.
Thus it will be seen that in general, although there is an increase in sound transmission loss over this median frequency range when the thicknesses of the sheets are increased, the extent of this frequency range is reduced, and as a consequence of this it is in practice extremely difficult to construct a multiple glazing panel across which the mean sound transmission loss exceeds a given value. By way of example, the mean sound transmission loss across a known double glazing unit will not in general exceed 35 dB.